This invention relates generally to the metal treating art, and more particularly to the production of aluminum billets which may be subsequently processed in an extrusion operation to form elongated shapes such as electrically conductive rod and wire, as well as other articles.
Cast metal billets, particularly billets cast from aluminum and aluminum alloys, are generally heat-treated prior to extrusion in order to homogenize, or render more uniform, the crystalline structure thereof. This operation is, of course, well known to those skilled in the art, and the particular treating temperatures and immersion times are functions of the particular billet size and constituency.
Subsequent to the homogenizing operation, the billets must be quenched in order both to facilitate handling as well as to improve the metallurgical properties thereof. A primary concern in the quenching of billets that are to be subsequently extruded, however, is that the billets do not warp to such an extent that they cannot be processed through the dies of an extrusion apparatus. To this end, certain allowable warp standards or straightness tolerances have been established in the industry, as follows:
51/8 INCHES TO 8 INCHES DIAMETER BILLETS --
Maximum allowable lateral bow is 1/8 inch in a PA1 Length of 31/2 times the billet diameter; PA1 Maximum allowable lateral bow is 3/16 inch in a PA1 Length of 31/2 times the billet diameter.
9 INCHES TO 12 INCHES DIAMETER BILLET --
As the degree of warp is, at least to some extent, a function of the rapidity of the quenching operation, slow air cooling has been used in the prior art as one method of limiting the degree of warp. However, air cooling is, of course, very time consuming and does not lend itself to a continuous production operation.
At the other extreme, some prior art systems utilized a complete immersion of the billets in a water bath. The drastic temperature gradients, however, invariably caused severe bending and bowing of the billets.
Between these extremes, other prior art quench systems utilized batch-cooling techniques in which a mass of billets was sprayed with water while large fans passed air thereover. This method did not cool the metal uniformly and thus resulted in billets of poor quality that, in many instances, had to be rejected. Moreover, spraying of water against only one side of the billets increased the temperature gradients thereacross and thus further exacerbated the warp problem.